Σχόλια 0

Το κείμενο του εγγράφου

The object of this experiment is to study the laws of electromagnetic induction(Faraday's law and Lenz's law).

THEORY:

Electromagnetic induction is the process in which a magnetic field causes or

INDUCES

an electric field. However, a "static" magnetic field will not do this; it takes a

CHANGING

magnetic field. A changingB-field can be produced in several ways: (1) the currentproducing theB-field can change, (2) the source of the field (electromagnet or permanentmagnet) can be in motion, or (3) the frame of reference in which theB-field is being observedcan be in motion. The induced electric field exists only while the change is occurring. If there isno change, then there is no induction.

When there are conductors present, the induced electric field is usually described in terms of aninduced electromotive force (EMF). If the conductors form complete circuits, then the inducedEMF's produce induced currents. Faraday's law of induction

relates the induced electric field tothe changing magnetic field. For the case of a circuit,Faraday's law

can be written as

dtdEMF,

where

is the magnetic flux through the circuit. When theB-field is uniform and at right anglesto the areaA

bounded by the circuit, then the flux is simplyB

A. (For non-uniform fields, the fluxmust be calculated by integration.)

Often it is useful to distinguish between the case where theB-field passes through the areaA

inone direction (e.g., left to right) and the case where it passes through in the opposite direction(e.g., right to left). This can be done by letting the flux be positive in one case (it doesn't matterwhich) and negative in the other case. When this is done, a reversal of the field direction or arotation of the circuit by 180° relative to theB-direction will cause a sign change in the fluxthrough the circuit.

Often a minus sign is used in the Faraday's law equation which gives information about thedirection of the induced E-field and thus the sign (polarity) of the resulting induced EMF. Whencurrents are possible, Lenz's law can be used to predict the current direction. A useful version ofLenz's law

is:

The induced current will appear in such a direction that the magnetic flux of thefield caused by that current will try to cancel the flux change that produced thecurrent in the first place.

In other words, the magnetic effects of the induced current opposes the change in conditionswhich produced it.

For this experiment, the secondary (yellow) coil and the meter comprise the circuit in which theinduced EMF and induced current appear. The many turns in the coil magnify the small effectsthat would be observed with only a single turn.

Electromagnetic Induction

2

PROCEDURE:

(In order to be sure that your observation of an effect was correct, you should repeat any step inthis procedure as many times as you need to.)

I. Bar magnet at rest inside the coil.

1.

Place the bar magnet inside the secondary coil (yellow coil with many turns) with the Northpole inside.

2.

Connect the multimeter in DC volt mode across the terminals of the secondary coil and noteany voltage that occurs as the last connection is made.

a.

Was there a (non-zero) reading? __________

b.

Was your observed result expected? Explain.

II. Bar magnet moved.

1.

Move the magnet (with theNorth

pole in the secondary coil) about half wayout

of the coiland thenremove

the rest quickly (with a jerk).

a.

Which sign did the voltmeter give (+ or-)? __________

b.

Which way did the INDUCED magnetic field generated by the INDUCED voltage point(i.e., which end of the secondary coil was the North pole )? __________

c.

Did this induced field oppose or support the field of the bar magnet? __________

d.

Comment on this observation in relation to Lenz’s Law.

2.

Does the speed at which you remove the magnet affect thesign(polarity) of the inducedvoltage? __________

3.

Does the speed at which you remove the magnet affect the maximumsize

of the inducedvoltage? __________

4.

Now

insert

the magnet (North pole in) about half way into the coil with a quick movement.

a.

Which sign did the voltmeter give (+ or-)? __________

b.

Which way did the INDUCED magnetic field generated by the INDUCED voltage point(i.e., which end of the secondary coil was the North pole )? __________

c.

Did this induced field oppose or support the field of the bar magnet? __________

d.

Comment on this observation in relation to Lenz’s Law.

5.

Use your knowledge of theB-field around a bar magnet to answer the following.

a.

Did you increase or decrease the flux through the coil in step 1 of this section? ______

pole of the magnet (as in step 4) and note thesign of the induced voltage.

a.

Which sign did the voltmeter give (+ or-)? __________

7.

Quicklyremove

theSouth

pole of the magnet.

a.

Which sign did the voltmeter give? __________

8.

Are the observations of steps 6 and 7 consistent with your statement in step 5 and Lenz’sLaw? __________ Explain.

III. Electromagnet moved.

The primary coil, which fits inside the secondary, functions as an electromagnet. It produces a

B-field with geometry similar to that of a permanent bar magnet, but the field magnitude isproportional to the current and the field direction reverses if the current direction is reversed.

1.

From an inspection of the direction of the windings of the primary (blue, fewer turns, thicker)coil, determine which of its two terminals should be connected to the positive terminal of thepower supply in order to make the bottom of the primary coil a North magnetic pole. (HINT:Consider the primary coil to be a solenoid.)

2.

Connect the primary coil to a power supply with a switch in series so that the current to thecoil can be turned on and off by closing and opening the switch. Leave the secondary coilconnected to the voltmeter.

3.

Place the primary coil (blue)inside the secondary (yellow) coil. Close the switch and adjustthe voltage control to thelowest possible setting

that will give about 1 amp in the primarycoil.

4.

Remove

the primary coil (electromagnet) from the secondary coil with a small jerk.

a.

Whatis the sign of the induced voltage on the voltmeter? __________

b.

Is this consistent with what happened when you removed a North pole from insidethe secondary coil in Part II? __________

__________

5.

Insert

the primary coil (electromagnet) into the secondary coil with a quick movement.

a.

What is the sign of the induced voltage on the voltmeter? __________

b.

Is this what you expected? __________

Electromagnetic Induction

4

IV. Current in the electromagnet switched off and on.

1.

Set the current in the primary coil to about 1 amp as in Part III. Place the primary coil insidethe secondary coil.

2.

Open

the switch. What is the sign of the induced voltage on the voltmeter? _________

3.

Close

the switch and again record the sign of the induced voltage.

__________

4.

Explain why openingand closing the switch causes the sign of the induced voltage on thevoltmeter to change and why it gave the signs that it did. Your explanation must contain thewords and phrases:

the voltage controlknob. Disconnect the voltmeter from the secondary coil andreplace the meter with theanalog galvanometer. (The galvanometer is basically an ammeter.)

2.

Open and close the switch and note the size of the induced current. Now place the

iron

(heavier) rod completely inside the primary coil. Open and close the switchagain.

a.

Has the size of the induced current significantly increased? __________

b.

Explain this observation.

3.

Remove about a fourth of the iron rod from the coil. Open and close the switch and note thesize of the induced current. Repeat with about half ofthe rod removed and again with aboutthree quarters removed.

a.

What happens to the size of the induced current as more of the iron rod isremoved from the coil? __________

b.

Is this consistent with the preceding observation made in step 2? __________

4.

Replace

the iron rod with a

pencil

and repeat step 2.

a.

Do you still get a significant increase in the induced current over what you hadoriginally with just air inside the coil? __________

5.

Replace the pencil with the

magnesium

rodand repeat step 2. (The magnesium rod is muchlighter in weight than the iron bar).

a.

Do you still get a significant increase in the induced current over what you hadoriginally with just air inside the coil? __________

b.

Does a magnet attract theiron

bar? _________

c.

Does a magnet attract thepencil?

d.

Does a magnet attract themagnesium

bar? _________

e.

Explain how all of these observations made in steps 4 and 5 relate using theprinciples of electromagnetic induction.

Electromagnetic Induction

5

VI. Effect of increasing current

1.

Use the same circuit as in part V

with the

iron

rod in completely inside the primary coil. Seewhat the level of current in the electromagnet (primary coil) does to the induced current in thesecondary coil as you open and close the switch to the electromagnet. Record yourobservations.

a.

Does the induced current in the secondary coil depend on the amount of current in theelectromagnet? __________

2.

If a more detailed study where made, we would find that the induced current in the secondarycoil depends linearly on the electromagnet current. Explain this linear dependence in terms ofFaraday’s Law and other relevant relationships.